In the prior art, nylons are typically prepared by polycondensation of diamine with dicarboxylic acid. Generally, in order to keep the molar amount of diamine as equal to that of dicarboxylic acid as possible, a nylon salt is first prepared from diamine and dicarboxylic acid, which is then subjected to a polycondensation process. Therefore, as precursor of nylon polymer, the nylon salt has a great effect on the polymer in quality. Qualified nylon salt is a base for producing a nylon polymer of high quality.
In the present market, nylon salt comprises aqueous solution of nylon salt and nylon salt in solid state. The aqueous solution of nylon salt is being gradually replaced by the solid nylon salt since the former has high water content, which leads a high cost for long distance transportation, and has low stability as well. In contrast, the solid nylon salt has high stability and low transport cost, and is therefore the main product in domestic and abroad markets.
Solution process is usually used in the known preparation of nylon salt, comprising dissolving or dispersing diamine and dicarboxylic acid in water separately, and mixing both of them for a neutralization reaction. The end-point of the reaction is determined by the pH of the mixture solution. After completion of the reaction, nylon salt is obtained by the steps of separation, purification and drying in sequence.
In the formation of a nylon salt, diamine and dicarboxylic acid are in equimolar amount mixed and reacted to form the nylon salt. With the reaction processing, the amount of diamine in a free state reduced gradually until the completion of the reaction. If the diamine and the dicarboxylic acid were not mixed evenly in the reaction process, the content of the unreacted diamine in nylon salt product will be high, which will bring about an important influence on the quality of the nylon salt. In addition, when exposed to the air, the amine is not chemically stable and is subjected to some side reactions under heat treatment or contacting with oxygen, which may cause the quality of nylon salt to be deteriorated and to become yellow in color. Furthermore, the changes described above will significantly bring about a negative influence on the production of nylon. In order to obtain a nylon salt with high quality, therefore, the content of free diamine remained in the nylon salt should be extremely low or nonexistent.
The Chinese patent application publication CN1887841A disclosed a solution process for preparing nylon salt, in which water is used as solvent. In addition to water, such organic solvents as alcohol or N,N-dimethylformamide (DMF) can also be used as solvent in the solution process for preparing nylon salt. The Chinese patent application publication CN101456804A disclosed use of N-methyl pyrrolidone or DMF as solvent in the solution process for preparing nylon salt. And dimethyl sulfoxide (DMSO) can be used as solvent in the solution process for preparing nylon salt, as disclosed in the Chinese patent application publication CN101880235A.
However, in these existing solution processes for preparing nylon salt, there exist the following defects. In case of using water as solvent to prepare nylon salt, it is difficult to obtain solid nylon salt by cooling-crystallization efficiently, because nylon salt has high solubility in water. Although concentrating-crystallization is alternative, it may cause the quality of nylon salt to become worse by reason of high temperature. Therefore, the preparation of nylon salt from its aqueous solution will give a high cost, and the product as obtained is not stable in quality. Further, in case of using organic solvent to prepare solid nylon salt, it is readily to cause the organic solvent to be remained in nylon salt, and the recovery of the organic solvent will bring about a high energy consumption, heavy pollution and high cost. Thus, the process for preparing nylon salt by way of organic solvent is also undesirable.
The Chinese patent application publication CN103201314A disclosed a method for preparing nylon salt under low water content, in which a diamine in liquid is reacted with a dicarboxylic acid powder. However, this reaction should be carried out at a temperature of 100 to 210° C. which is higher than the melting point of pentanediamine, and is not a mild reaction. And the nylon salt obtained by this high temperature reaction will render the nylon product produced in the subsequent polymerization yellowing, which would bring an adverse effect on the color of the nylon product. Besides, there are very strict limitations to the reactive system, the manner of stirring, the reaction temperature, the mode for adding dicarboxylic acid and so on, and the method is accordingly a complicated process. As such reaction is carried out between a liquid reactant and a solid reactant, some free dicarboxylic acid is inevitably remained in the final product.
In a word, it is difficult for the conventional preparation to directly obtain a nylon salt with low residual diamine and dicarboxylic acid content and without adverse effect on the subsequent polymerization producing nylon.